High temperature primary surface recuperator air cell

ABSTRACT

Primary surface recuperators generally undergo severe thermal and pressure cycles. Thermal cycling tends to cause the primary surface recuperator to expand along a central axis. However ducting connected with the primary surface recuperator tends to limit its expansion. Constructing bars in cells of the primary surface recuperator from the same material as the ducting tends to reduce thermal stresses that may otherwise result from difference in thermal expansion.

TECHNICAL FIELD

[0001] This invention relates generally to a recuperator and more particularly to a cell of the recuperator.

BACKGROUND

[0002] Many gas turbine engines use a heat exchanger or recuperator to increase the operating efficiency of the engine by extracting heat from the exhaust gas and preheating combustion air. Typically, a recuperator for a gas turbine engine must be capable of operating at temperatures of between about 500 C. and 800 C. and internal pressures of between approximately 140 kPa and 1400 kPa. During operation, the recuperator experiences repeated cycles of starting and stopping of the gas turbine engine.

[0003] An example of such a recuperator is disclosed in U.S. Pat. No. 5,060,721 issued to Charles T. Darragh on Oct. 29, 1991. Such recuperators include a core which is commonly constructed of a plurality of relatively thin flat sheets having an angled or corrugated spacer fixedly attached therebetween. The sheets are joined into cells, sealed and form passages between the sheets. These cells are stacked or rolled and form alternate air (recipient) cells and hot exhaust (donor) cells.

[0004] During operation, hot exhaust gas expands through a turbine turning a shaft connected with an air compressor. Compressed discharged air from the compressor passes through the air cell while hot exhaust gas flows through the hot exhaust cells. The exhaust gas heats the sheets and the spacers of the hot exhaust cells. Through conduction, heat transfers to the sheets and spacers of the air cells and ultimately the compressed air.

[0005] U.S. Pat. No. 5,918,368 issued to Ervin et al. on Jul. 6, 1999 improves reliability of the recuperator by making each air cell as an individual unit. Each air cell is made of a pair of primary sheets separated by a plurality of bars and a pair of guide strips. Making each cell as an individual unit improves reliability of the air cells. The hot exhaust cells are formed by connecting two air sells separated by a pair of gas guide strips. Generally, a plurality of cells are attached to form a recuperator core.

[0006] Severe environments in gas turbine engines increase stresses in connections between various components. As mentioned above, operating the gas turbine engine increases both temperatures and pressures in both the recuperator core and ducting causing both to expand. Further, these pressures and temperatures are cyclic and may lead to increased loading especially at connections where components have different thermal characteristics such as thermal expansion.

[0007] The present invention is directed to overcoming one or more of the problems as set forth above.

SUMMARY OF THE INVENTION

[0008] In one aspect of the present invention a cell for use with a recuperator has a first sheet and a second sheet having generally equivalent dimensions. A bar attaches between the first sheet and the second sheet. The bar is made of a second material having a coefficient of thermal expansion generally equivalent with a duct.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a partially sectioned side view of a gas turbine engine including a primary surface recuperator embodying the present invention;

[0010]FIG. 2 is a sectioned view of the recuprator taken along line 2-2 looking at a recipient side of a sheet as is embodied in the present invention; and

[0011]FIG. 3 is a view of a cell assembly.

DETAILED DESCRIPTION

[0012] Referring to FIG. 1, a gas turbine engine 5 is shown having a primary surface recuperator 10 with a plurality of cells 12. The primary surface recuperator 10 has a first surface 16 and second surface 18. An air inlet duct 20 and air outlet duct 21 are connected proximate the first surface 16 and second surface 18 respectively. Each of the plurality of cells 12 are separated by a respective gas guide strip 22.

[0013] Further defining the invention, FIGS. 2 and 3 shows one of the plurality of cells 12 having a first sheet 26, a second sheet 28, an air guide 30, an exhaust guide 32, a first air bar 31, a second air bar 33, a first gas bar 34, and a second gas bar 35. The first sheet 26 and second sheet 28 each have generally identical dimensions. In this application, the first sheet 26 and second sheet 28 have central portion 36 generally trapezoidal in shape separating a first wing portion 38 from a second wing portion 40. The central portion 36 is corrugated while the first wing portion 38 and second wing portion 40 are generally flat with respect to the central portion 36. The first sheet 26 and second sheet 28 are made from a first material that is a thermally conductive, oxidation resistant material such as stainless steel.

[0014] The first gas bar 34 and second gas bar 35 are attached to the first sheet 26 in some conventional manner such as tack welding or adhesive. The air guide 30 is positioned between the first sheet 26 and second sheet 28 on the first wing portion 38 and second wing portion 40 opposite the gas bars 34, 35. In this application, the air guide 30 has a plurality of passages 42 generally perpendicular to the corrugations forming a Z-flow path. The passages 42 may also form other flow paths such as a C-flow wherein the first wing portion 38 and second wing portion 40 would be mirror images of one another. While the passages 42 in this application are shown as trapezoidal, any conventional shape may be used. The air guide 30 is made from an oxidation resistant material such as stainless steel, ceramic, or other conventional materials that maintain their mechanical strength in the gas turbine engine environment.

[0015] Similarly, the exhaust guide 32 is positioned on the first wing portion 38 and second wing portion 40 opposite the air guide 30. In this application the exhaust guide 32 has a plurality of passages 43 generally parallel with the corrugations. Like the air guide 30, the exhaust guide is made from an oxidation resistant material such as stainless, steel, ceramic, or other conventional materials that maintain their mechanical strength in the gas turbine environment.

[0016] The first air bar 31 and second air bar 33 further separate the first sheet 26 from the second sheet 28 by running along a periphery of the first sheet 26 and the second sheet 28. The bars 31, 34 sealingly connects the first sheet 26 and second sheet 28 through some conventional manner such as welding leaving only the passages 42 through the cell 12 between sheets 26 and 28. In the present embodiment, the first air bar 31 and second air bar 33 are L-shaped. Alternatively, the bars may be of different shapes so long as air may be directed through the sheets 26, 28 along the corrugations over some predetermined length. The present invention requires that at least the first air bar 31 adjacent the air outlet duct 21 is made from a material having superior oxidation resistance at high temperatures such as a nickel based alloy and the material has a coefficient of thermal expansion similar to that of the air outlet duct. Optionally, the second air bar 33 may have a duct tab portion 48 preferably near the air outlet duct 20. For simplicity all of the bars 31, 33, 34, and 35 may be made of the same material.

[0017] The air inlet duct 20 is connected to the primary surface recuperator 10 proximate the second surface 18. The air outlet duct 21 is connected to the primary surface recuperator 10 proximate the first surface 16. In one embodiment of the present invention, the air outlet duct 21 is welded to the duct tab portion 48. The air outlet duct 21 is made from a first material having similar thermal characteristics as the duct tab portion 48 such as oxidation resistance, thermal conductivity, and coefficient of thermal expansion. Preferably the air outlet duct 21 is make of a second material such as nickel based alloy. In this application the second material has a lower coefficient of thermal expansion than the first material. Alternately, both the air inlet duct 20 and the air outlet duct 21 may be attached to duct tab portions 48 proximate the inlet portion 14 and outlet portion 15 respectively.

INDUSTRIAL APPLICABILITY

[0018] As exhaust gases pass through the primary surface recuperator 10, separate components begin to expand due to increasing temperatures. Each component in the primary surface recuperator 10 may be constrained by interactions with other components.

[0019] The air outlet duct 21 at a minimum must be made to withstand the extremes of the gas turbine engine environment. Using the nickel based alloy or similar material insures good oxidation resistance in the gas turbine environment. Making the bar 34 of the same material increases compatibility of axial thermal expansion between the primary surface recuperator 10 and the air outlet duct 21. Increased compatibility of axial thermal expansion reduces thermal strains that may otherwise exist if the primary surface recuperator 10 and air outlet duct 21 expanded at different rates.

[0020] In the cells 22, only the bar 31 needs to be made of the nickel based alloy or similar material. The air bars 31,33 determines axial expansion of the first sheet 26 and second sheet 28. Allowing the second air bar 33 to be made of the first material having a greater thermal expansion may reduce thermal stresses. The second air bar 33 is exposed to lower temperatures and therefore not as likely to created undue expansion. Allowing the second air bar 33 to expand further at lower temperatures than the first air bar 31 increase likelihood of similar thermal growth. Further, the first sheet 26 and second sheet 28 must have good thermal conductivity. Thermal conductivity may not be a consideration in selecting proper materials for making the air outlet duct 21.

[0021] Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims. 

What is claimed is:
 1. A cell for use in a primary surface recuperator, said cell comprising: a first sheet being made from a first material; a second sheet having generally equivalent dimensions as said first sheet, said second sheet being made from said first material; and an air bar being attached between said first sheet and said second sheet, said bar being made from a second material; wherein said second material has a lower coefficient of thermal expansion than said first material.
 2. The cell for use in said recuperator as described in claim 1 wherein said second material is a nickel based alloy.
 3. The cell for use in said recuperator as described in claim 1 wherein said air bar being proximate an air outlet duct.
 4. The cell for use in said recuperator as described in claim 1 including a second air bar attached between said first sheet and said second sheet.
 5. The cell for use with said recuperator as described in claim 4 wherein said second air bar being made of said first material.
 6. The cell for use with said recuperator as described in claim 5 wherein said second air bar being made of said second material.
 7. The cell for use with said recuperator as described in claim 1 wherein said air bar further comprises a duct tab portion, said duct tab portion is adapted for connection with said air outlet duct.
 8. A recuperator comprising: a plurality of cells, each of said plurality of cells comprising a first sheet, a second sheet, said first sheet and said second sheet having generally equivalent dimensions, said first sheet and said second sheet being made of a first material, a bar separating said first sheet from said second sheet, said air bar being made from a second material; and a duct being connected with said plurality of cells about said bar, said duct being made of said second material.
 9. The recuperator as described in claim 8 wherein said second material is a nickel based alloy.
 10. The cell for use in said recuperator as described in claim 8 wherein said air bar being proximate said air outlet duct.
 11. The cell for use in said recuperator as described in claim 8 including a second air bar attached between said first sheet and said second sheet.
 12. The cell for use with said recuperator as described in claim 11 wherein said second air bar being made of said first material.
 13. The cell for use with said recuperator as described in claim 11 wherein said second air bar being made of said second material. 